Russian opposition activists, who led tens of thousands of people onto the streets in anti-government rallies in recent months, have merged two parties into one to strengthen their fight against President Vladimir Putin.

Mikhail Kasyanov, prime minister from 2000 to 2004 during Putin's first term as president but now a fierce critic, and former Deputy Prime Minister Boris Nemtsov, united their People's Freedom Party with former lawmaker Vladimir Ryzhkov's Republican Party of Russia (RPR) on Saturday.

"Our main goal is a change in the country's political course. This can be achieved only via free elections," Kasyanov said on the sideline of a conference where party delegates created the new party RPR-PARNAS.

The opposition got a boost last December when tens of thousands of Russians took to the streets against what they said were rigged parliamentary elections won by ruling United Russia.

Such numbers had not been seen since Putin's rise to power in the late 1990s.

Putin, who returned to the Kremlin in May for a six-year term, responded the protesters by promising modest electoral reforms including easing registration requirements to enable more parties to contest elections.

But he also approved legislation setting harsher fines, in some cases thousands of dollars, for participants in demonstrations which violate public order.

Last month the Justice Ministry gave the RPR permission to register after a long court battle that began when it lost its legal status in 2007 as Putin tightened his grip on electoral politics.

The RPR was always a small force but its members had a dozen parliamentary seats in the early 1990s which it gradually lost during Putin's first term as president.

Opposition leaders hope to gain a foothold in future regional and municipal elections.

Earlier this week, police searched the homes and offices of several opposition leaders, including Nemtsov, after series of mass protests against the rule of Putin.

Nemtsov compared Putin's actions against the opposition to the that of the leader of neighbouring Belarus, Alexander Lukashenko.

"This is the Lukashenisation of Russia. This machine of violence will gain more strength," Nemtsov said during the party meeting.